Communication systems using modulated light waves transmitted through a light transmitting optical fiber are becoming of increasing commercial importance. In these systems an electrical signal causes modulation of the output of a light source which is coupled into an optical fiber and transmitted through the fiber to another location. A light detector detects the transmitted light and converts the incident modulated light signal to a modulated electrical signal corresponding to the original electrical signal impressed upon the light source. These systems offer the advantages of large signal bandwidth, immunity from electrical noise, and small size. To be reliable the components of such systems must be shielded against the severe environmental conditions found in transmission and receiving installations and the distances between such installations.
The light detector used in such systems is typically a p-i-n or avalanche photodiode. The photodiode is encased in a housing which includes a header on which the light detector is mounted. Electrical leads, required for biasing the detector and transmitting the electrical signal corresponding to the detected light signal, are inserted through the header. The openings in the header, through which these leads are inserted and then sealed, have a glass to metal, an epoxy resin, or solder (if electrical contact to the header is desired) seal, to form a hermetic seal between the electrodes and the header. A cap, with a large area window in it to permit entry of light into the housing, is soldered or welded to the header to form a hermetic seal therebetween. The window is sealed to the cap using a glass to metal or an epoxy resin seal. Collins et al, U.S. Pat. No. 3,805,347 issued Apr. 23, 1974, teach a housing for use with a light emitting diode. Hacskaylo, U.S. Pat. No. 3,946,416 issued Mar. 23, 1976, teaches a low impedence housing for use with a laser diode. In both of these patents a large area window in the housing cap, essentially identical to that used for a prior art light detector housing, is shown.
Light detectors mounted in such prior art housings are not suitable for use in a fiber optic communication system such as described above since the window of the package will be typically at least 0.1 cm in thickness, and the light detector will be mounted some distance behind the inside surface of the window, typically 0.05 cm or more. A light beam exiting from a fiber optic cable butted against or in close proximity to the window will spread with a full angle corresponding to twice the numerical aperture of the fiber, typically 22 to 88 degress. The light beam spreads as it propagates through the window and the interior of the housing toward the light detector. The user is then faced with one of two alternatives; losing part of the detected signal, since the beam spreads and is not completely intercepted by the light detector; or, increasing the diameter of the detector to intercept the entire beam. The first alternative is unattractive because it requires either a larger amount of light to be initially coupled into the fiber to attain a given signal to noise ratio, or, correspondingly it decreases the length of a particular communication system. The second alternative is unattractive since, as the area of the detector increases, its capacitance also increases, which increases the detector noise current and time constant.
A third possibility is to remove the cap and window from the device completely. This alternative is also unattractive since the hermetic seal of the housing against the surrounding environment is destroyed.
Thus, it would be desirable to have a light detector housing in which all light emitted from an optical fiber intercepts the detector, with the detector size remaining comparable to the size of the fiber.
Goell et al, U.S. Pat. No. 4,065,203 issued Dec. 27, 1977, teach the use of an optical fiber contained in a support tube which is inserted through an opening in the cap of a laser diode housing. The tube is glued both to the mount of the laser diode and to the cap of the laser diode housing, extending through this housing into the region outside the cap. Goell et al do not teach the use of a fiber, alone, bonded to the cap nor do they teach a method for obtaining a hermetic seal of the fiber to the cap.